This invention relates to a microelectromechanical resonator architecture; and more particularly, in one aspect, to a resonator architecture including a plurality of in-plane vibration microelectromechanical resonators that are mechanically coupled and may provide a differential signal output.
Generally, high Q microelectromechanical resonators are regarded as a promising choice for integrated single chip frequency references, filters and sensors. In this regard, high Q microelectromechanical resonators tend to provide high frequency outputs that are suitable for many high frequency applications requiring compact and/or demanding space constrained designs. However, when the output frequency of the resonator is “pushed” higher while being scaled smaller, the motional resistance of such devices tends to dramatically increase. As such, the motional resistance of such resonators should be minimized and/or reduced in order to match impedance and maximize the amplitude of the output signal.
There are a number of conventional techniques to decrease the motional resistance, including: (1) adjusting (i.e., increasing) the DC bias voltage, (2) adjusting (i.e., decreasing) electrode-to-resonator gap space, and (3) implementing an array of mechanically coupled identical resonators. The first two techniques are well known and often effective, but include significant shortcomings, for example, an increase in nonlinearity as a result of increasing the DC bias and scaling the gap space. Moreover, to achieve a very narrow gap often leads to a significant increase in the complexity of the fabrication processes.
An array of identical mechanically-coupled resonators has been proposed to decrease motional resistance while improving linearity. (See, for example, U.S. Pat. No. 6,628,177 and “Mechanically Corner-Coupled Square Microresonator Array for Reduced Series Motional Resistance”, by Demirci et al., Transducers 2003, pp. 955-958). However, care must be taken during the fabrication processes. That is, such resonators often are required to fabricated with high precision in order to manufacture an array of identical mechanically-coupled resonators with precisely identical resonant frequency.
Moreover, conventional microelectromechanical resonator arrays include both out-of-plane motion as well as in-plane motion type resonators. The out-of-plane vibration mode type resonator is not a commonly employed architecture. In this regard, out-of-plane vibration mode (i.e., transverse mode) requires a bottom driving and sensing electrodes at the cost of parasitic capacitance between drive/sense electrodes and the substrate. Such capacitance may lead to a higher noise floor of the output signal (in certain designs). In addition, the resonator requires at least one additional mask to fabricate, as compared to the in-plane vibration resonator, in order to define the drive/sense electrode.
Although conventional microelectromechanical resonator arrays propose an in-plane vibrating mode type resonator, the inputs and outputs of such microelectromechanical resonators are not fully differential. Accordingly, such conventional microelectromechanical resonator arrays tend to be more susceptible to noise since the lack of full differential signaling may lead to poor immunity to noise on the input signals and/or the output signals.
Thus, there is a need for a system and technique to overcome the shortcomings of one, some or all of the conventional microelectromechanical resonator arrays. In this regard, there is a need for an improved microelectromechanical resonator array that includes relatively small motional resistance and good linearity, implements full differential signaling and/or possesses a high immunity to noise on the input signals and/or the output signals.